Problem: Solve for $n$ : $-1 = 20 + n$
Subtract $20$ from both sides: $ \begin{eqnarray} \\ \hphantom{-1 {- 20}} &\hphantom{=}& \hphantom{n} \\ -1 &=& 20 + n \\ \\ {-20} && {-20} \\ \end{eqnarray} $ $ \begin{eqnarray} \\ -1 {- 20} &=& n \\ \end{eqnarray} $ Simplify. $ n = -21$